In today's information age, communication devices, such as computers and computer peripherals, are often internetworked over a communication network. A common networking model routes packets of information within the communication network using a networking protocol such as the Internet Protocol (IP) or other network layer protocol. Some networking protocols, such as IP, are considered to be “connectionless” networking protocols. In a connectionless networking protocol, each packet of information includes a network layer address, and each router forwards the packet of information based upon the network layer address according to predetermined signaling and routing protocols, such as the Open Shortest Path First (OSPF) protocol, the Routing Information Protocol (RIP), Hello, Border Gateway Protocol (BGP), RSVP, or other routing protocol.
Thus, each router makes an independent forwarding decision for the packet based upon the network layer address. Essentially, each router partitions the entire set of network layer addresses into a number of Forwarding Equivalence Classes (FECs), and each FEC is mapped to a particular outgoing path (or set of paths, in the case of multi-path routing) based upon the routing protocol. The router determines an FEC for each packet of information based upon the network layer address of the packet, and forwards the packet of information to the corresponding outgoing path (or set of paths).
Network layer routing requires each router to process each packet of information at the network layer. This is an expensive and time-consuming operation that limits the performance of some routers and even prevents certain devices that do not support the networking protocol from performing routing and other functions on the packets.
Label switching can be used to eliminate the network layer processing by certain devices in the communication network. Label switching enables a packet to be transported across a network domain (referred to hereinafter as an “autonomous system” or “AS”) using labels rather than the network layer address. Specifically, a Label Switched Path (LSP) is established from an ingress point border device to an egress point border device in the AS. The LSP traverses a number of label switching devices. Each label switching device assigns a short, fixed-length value (i.e., a “label”) to each FEC that is supports. When the packet enters the ingress point border device, the ingress point border device uses the network address to assign the packet to a particular FEC, and inserts the corresponding label into the packet, specifically within a packet header. Each subsequent label switching device along the LSP uses the label in the packet to determine the next hop FEC for the packet, and replaces the label in the packet with a new label corresponding to the next hop FEC for the packet. The egress point border device removes the label from the packet. Thus, only the border devices process the packet at the network layer, while the intermediate devices process the packet based upon the label only.
The Internet Engineering Task Force (IETF) Multi-Protocol Label Switching (MPLS) working group has defined an MPLS architecture for utilizing label switching for internetworking. MPLS is considered to be “multi-protocol” because it can be used with any network layer protocol, and is not limited to IP. An MPLS framework is described in an IETF Internet Draft document entitled A FRAMEWORK FOR MULTIPROTOCOL LABELING SWITCHING, which is referenced as draft-ietf-mpls-framework-05.txt (September 1999), and is hereby incorporated by reference in its entirety. The MPLS architecture is described in an IETF Internet Draft document entitled MULTIPROTOCOL LABEL SWITCHING ARCHITECTURE, which is referenced as draft-ietf-mpls-arch-06.txt (August 1999), and is hereby incorporated by reference in its entirety.
In order to establish and remove LSPs, the various label switching devices exchange label switching information using a signaling protocol. One such signaling protocol that is used specifically for exchanging label switching information is the Label Distribution Protocol (LDP). LDP is described in an IETF Internet Draft document entitled LDP SPECIFICATION, which is referenced as draft-ietf-mpls-ldp-04.txt (May 1999), and is hereby incorporated by reference in its entirety. Label switching information can also be carried (“piggy-backed”) in other signaling and routing protocols, such as OSPF, IS-IS, and RIP.
Each label switching device maintains mapping information for mapping each FEC to a corresponding label. The label mapping information is typically maintained in the various forwarding/routing tables maintained by the label switching device. It is common for the label switching device to maintain a forwarding table for each incoming interface and a forwarding table for each outgoing interface. The label mapping information maintained by the label switching device in the incoming forwarding tables enables the label switching device to quickly forward received packets that include label switching information. The label mapping information maintained by the label switching device in the outgoing forwarding tables enables the label switching device to insert label switching information into packets. For convenience, a forwarding table that includes label mapping information may be referred to as a label information base (LIB).